Add ability to @Bind to request's attachments.

[Aldo111]

It'd be great to have syntactic sugar to do something like below to clearly access middleware data passed to endpoint controller.

@Bind.attachment('user_id')

[sjapps]

This would be cool. Currently we have a middleware controller that checks for the auth and then passes on the userId to the respected controller and have to manually parse it. Not a big deal but would be cool to have a bind for it.